This invention relates to the production of building material, and more particularly to a process of producing a building product of gypsum, particularly a gypsum slab, preferably with a volumetric weight of &lt;0.8 g/cm.sup.3 for the dry building product, in which process the quantity of mixing water required for rehydration and molding is preferably or exclusively introduced into the gypsum by means of water saturated porous particles added in a distributed form.
A process of this kind, together with the relevant apparatus, is already known from U.S. Pat. No. 3,809,566. In this known process the gypsum is given an addition of crushable water saturated particles from which the water required for the rehydration is removed by subjecting the mixture to a high pressure between 3.4 and 51 N/mm.sup.2. In practice, therefore, this method calls for correspondingly large presses and is not suitable for building products of low density. Furthermore, pressure-proof water-impregnated particles are not suitable for this process.
Industrial gypsums, which include semi-hydrate plasters and anhydrous plasters, are rehydrated, with the addition of water, to form dihydrates. In this process, semi-hydrate and anhydrite are first of all dissolved in the mixing water, forming solutions which are supersaturated in relation to the dihydrate and from which the gypsum rapidly crystallizes out into crystals which undergo felting. The rehydration is an exothermic process, for the reaction of plaster and water is accompanied by the release of energy.
The quantity of water which is required for complete rehydration and which is thus chemically bound depends on the degree of purity of the industrial gypsums concerned and is usually between 14 and 21%.
In the rehydration of the industrial gypsums to form dihydrate, however, the customary wet process, in which the gypsum is processed in a pulpy or liquid consistency, necessitates the adoption of a considerable quantity of surplus water; the required ratio of water to gypsum in hard mold gypsum, for instance, is about 0.4, while with building gypsum it amounts to 0.7-0.8. Some of the surplus mixing water already emerges during compression in the mold, while some of it, after the setting process, is present as free water in the dihydrate, and in prefabricated building elements, such as gypsum boards, gypsum cardboard slabs or gypsum fiber slabs, it is expelled by an operation involving the supply of heat, i.e. by industrial drying.
Attempts have already been made to limit this consumption of heat by ensuring that the surplus mixing water introduced into the mixture is kept to a minimum from the outset. This has been done, for example, by spraying water into a loosened current of pulverous plaster through a nozzle or by mixing the said current with fine ice crystals. In practice neither method proves satisfactory in every case, since in the spraying operation an additional mixture of gypsum and water on the mold base can hardly be avoided, while the use of ice crystals involves the consumption of additional heat.